The objectives of the study are to examine level of performance and pattern of neuropsychological functioning of children following treatment and recovery from Reye's Syndrome. In the first two years of a proposed three year project, 34 children with Reye's Syndrome and 18 matched controls have been seen for neuropsychological testing. Consistent diagnostic and treatment procedures were utilized in the care of Reye's patients, under the supervision of the medical member of the research team. Neuropsychological testing consisted of age appropriate versions of the Halstead and Reitan tests plus the semi-automated motor accuracy and speed tests. Assessments were scheduled to be completed within 30 days of the diagnosis of Reyes, 12 and 24 months later. The purpose of repeated testing of the same subjects is to describe any changes that may occur over time in behavioral performance among children varying in severity of initial illness. In the third year of this research program, new subjects will be added to the study and follow-up re-evaluation of previously enlisted subjects will continue.